Check the lug nuts for looseness. Could also be a bent wheel or out of round tire. I can tell you that Firestones are bad about having heavy spots in them, especially thir truck tires. Another thing to check is runout of the mounting flanges of the axle shafts and front hubs. If there's too much runout, it will cause a vibration. Alignment has been mentioned, and while it is a good idea for maximum tire life, alignment will not cause or cure a vibration.
